1. Field of the Invention
The present invention relates to a rotary operation type variable resistor suitable for use in an electronic apparatus which is used in a dusty and moist environment and which, therefore, needs high tightness, e.g., a throttle position sensor provided in a motor vehicle to perform a throttle control by adjusting the opening of a fuel injection valve according to the amount of depression of an accelerator pedal.
2. Description of the Related Art
FIG. 11 is a cross-sectional view of an example of a conventional rotary operation type variable resistor of this kind, which has been proposed by the applicant of the present invention. As shown in FIG. 11, the rotary operation type variable resistor has a case 20 which forms an outer shell of a throttle position sensor, and which is open at its upper and lower ends. A partition wall 21 is formed inside the case 20, and a bearing 22 is inserted in the partition wall 21. A sliding-element supporting member 23 and a resistor base 24 are successively inserted into the case 20 through the lower opening of the case 20. A shaft portion 23a of the sliding-element supporting member 23 is passed through the bearing 22 to reach the upper opening of the case 20. Sliding elements 25 are attached to a lower end portion of the sliding-element supporting member 23. The sliding elements 25 resiliently contact a resistor pattern 26 formed in a printing manner on a surface of the resistor base 24. The resistor base 24 is fixed inside the case 20 by a stepped portion formed in an inner wall of the case 20 and by a lid member 27 attached to the lower opening end of the case 20. On the other hand, at the upper opening end of the case 20, an operating lever 28 is fitted around the shaft portion 23a of the sliding-element supporting member 23 projecting out of the bearing 22. The end of the shaft portion 23a is caulked from the state indicated by the double-dot-dash line in FIG. 11 to the state indicated by the solid line in FIG. 11, thereby fixing the operating lever 28 on the sliding-element supporting member 23.
When the case 20 of the rotary operation type variable resistor thus constructed is mounted on a frame of a motor vehicle, the operating lever 28 is connected to an actuator of a throttle valve. As the operating lever 28 is rotated according to a throttle position or the like, the sliding-element supporting member 23 connected to the operating lever 28 is rotated together with the operating lever 28. The sliding elements 25 that resiliently contact the resistor pattern 26 on the resistor base 24 are thereby caused to slide on the resistor pattern 26, thereby changing the resistance of the variable resistor. From the resistance value, the throttle position or the like can be detected. The attachment structure of the case 20 is as described below. The throttle valve and other parts are accommodated in the frame having a hole. The case 20 is partially inserted into the hole so that the operating lever 28 faces the throttle valve. Thereafter, the case 20 is fixed on the frame by using screws or other means. At this time, the lid member 27 attached to the case 20 faces outwardly opposite from the frame, so that it can be exposed to dust, muddy water and the like. To prevent such extraneous materials from entering the case 20, joint cavities formed by portions of the lid member 27 and the case 20, and the like are filled with a sealing adhesive 29.
A known product designed as the above-described rotary operation type variable resistor has a hole formed in the resistor base 24 at a virtual center of the same corresponding to the center of rotation of the shaft portion 23a, and has the resistor pattern 26 formed by printing with this hole used as a reference portion. Also, a plurality of projections are formed on the opening peripheral end of the case 20. Notches formed in the peripheral end of the resistor base 24 are positioned on the projections. The size of portions of the opening peripheral end other than the projections is selected so that the peripheral end of the resistor base 24 can be press-fitted into the opening. This variable resistor is assembled as described below. The sliding-element supporting member 23 on which the sliding elements 25 are fixed is inserted and rotatably retained in the bearing 22 that has been inserted into the case 20. The operating lever 28 is then attached to the shaft portion 23a. Thereafter, the resistor base 24 is positioned on the case 20, and its peripheral end is press-fitted into the opening. Further, the lid 27 is placed so as to cover the resistor base 24, and the gap between the lid 27 and the case 20 is thereafter filled with adhesive 29 to fix the lid 27 in the opening of the case 20.
The bearing 22 of this kind of conventional rotary operation type variable resistor as a product is ordinarily formed in such a manner that a cylindrical bearing piece made of brass is integrally formed by an insert molding technique in the bearing hole formed in the case 20, and the inner circumferential surface of the cylindrical bearing piece is machined with certain accuracy necessary for the desired bearing performance. The shaft portion 23a of the sliding-element supporting member 23 is fitted in the bearing thus formed.
The above-described conventional rotary operation type variable resistor requires, in the process of assembling the parts in the case 20, steps of inserting the sliding-element supporting member 23 into the bearing 22 through one opening of the case 20, fitting the operating lever 28 around the shaft portion 23a at the other opening, caulking the corresponding end of the shaft portion 23a to fix the operating lever 28 on the sliding-element supporting member 23, and successively inserting the resistor base 24 and the lid 27 in the opening through which the sliding-element supporting member 23 has been inserted. That is, it is necessary for the parts to be assembled by being passed through or fitted in the two openings of the case 20, and it is also necessary to insert a jig into the opening of the case 20 at the member 23 insertion side for the purpose of receiving a caulking force when the end of the shaft portion 23a is caulked. Thus, troublesome assembly operations are required. Moreover, it is necessary to tightly seal the opening end side of the case 20 to which the lid 27 for covering the resistor base 24 is attached. There is, therefore, a need to fill the joint cavities formed by the lid member 27 and the case 20 and the like with adhesive 29, resulting in a further reduction in the facility of the assembly work.
In the above-described conventional rotary operation type variable resistor, the resistor base is positioned only before being pressed fitted to the opening with its peripheral end deformed. Therefore, a misalignment occurs easily between the resistor base and the case due to variation in the amount of deformation when the resistor base is attached to the case. For this reason, it is difficult to accurately attach the resistor base to the case. On the other hand, it is necessary to set a certain amount of play between the positioning reference center hole and a printing mask positioning pin (positioning member), which is large enough to enable the center hole and the positioning pin to engage with and disengage from each other. Therefore, a center misalignment (printing misalignment) of the printed resistor pattern can occur easily and it is difficult to maintain the desired accuracy of the position of the resistor pattern on the resistor base. Thus, the accuracy of each of positioning between the resistor base and the case and positioning between the resistor base and the resistor pattern is considerably reduced. Consequently, it is difficult to maintain the desired accuracy of the positions of the sliding elements and the resistor pattern supported on the case, so that the linearity of the resulting resistance changing characteristic is low.
In the above-described arrangement, two reference portions or members, i.e., the one for positioning when the resistor pattern is printed and the other for positioning when the resistor base is mounted in the case, are used. In such a case, if an error occurs in the mounted position of the resistor base in the case due to a variation in size as described above, the amount of error cannot be grasped accurately. As a result, the size control at the time of manufacturing is complicated.
Further, in manufacturing the above-described conventional rotary operation type variable resistor, there is a need to newly prepare molds for forming the case according to varieties of the attachment portion, and a step of assembling various parts in the case is required at the time of assembly. Accordingly, it is necessary to change jigs suitable for assembly steps each time the case is changed, so that the assembly process is complicated. To solve such a problem, a construction may be adopted in which only the attachment portion is formed separately from the case. Then, a misalignment occurs between the centers of rotation of the operating lever and the throttle position if the attachment portion is not accurately attached to the case. In such a situation, it is difficult to achieve the desired linearity of the change the resistance value of the variable resistor with respect to the change in the throttle position.
Moreover, in the above-described conventional rotary operation type variable resistor, a cylindrical metal bearing requiring machining on its inner circumferential surface is adopted for the bearing structure for axially supporting the sliding-element supporting member. Troublesome machining steps are therefore required, so that the productivity is low. The manufacturing cost is increased thereby.